The invention relates to a particle-optical apparatus which includes
a particle source for producing a primary beam of electrically charged particles which travel along an optical axis of the apparatus under the influence of an acceleration voltage,
a specimen holder with a specimen location for a specimen to be irradiated by the primary beam,
a focusing device for producing a focus of the primary beam in the vicinity of the specimen location,
a beam deflection system for scanning the specimen by means of the focused primary beam,
detection means with a detector surface for detecting electrically charged particles originating from the specimen in response to the incidence of the primary beam, said detector surface being arranged behind the specimen location, viewed from the propagation direction of the primary beam,
a mask for the electrically charged particles originating from the specimen, which mask is arranged between the detector surface and the specimen location.
A particle-optical apparatus of this kind is known from the published European patent application no. 0 348 992.
Apparatus for scanning a specimen by means of a focused beam of charged particles are known as scanning electron microscopes. Generally speaking, two types of scanning electron microscope are known. Both types include a focusing device for producing a focus of the primary beam in the vicinity of the specimen location, usually referred to as the objective.
The objective in the first type consists of a generally known magnetic gap lens in which the gap for forming the focusing magnetic field is formed by two oppositely situated cylindrical magnet poles. In this apparatus the specimen is arranged between the magnet poles of the objective. An apparatus of this kind is arranged to achieve a very small image of the electron source at the area of the specimen (the scan spot), enabling a very high resolution to be achieved for the image of the specimen. Such apparatus satisfies very severe requirements as regards stability, notably in respect of the mechanical as well as the electrical facilities, for example low sensitivity to vibrations of the specimen and stability of the accelerating voltage and the power supplies for the lenses. Moreover, such apparatus are arranged to produce the high accelerating voltages required for the high resolutions, which voltages amount to an order of magnitude of several hundreds of kV. Preparing the specimens so that they are suitable for analysis in such apparatus is a comparatively complex operation and the introduction of these specimens into the apparatus and the analysis therein is time-consuming. These apparatus, generally known as transmission electron microscopes (TEMs) are complex and expensive because of the severe resolution requirements.
In the second type of scanning electron microscope the objective may also consists of a generally known magnetic gap lens or a monopole lens, the specimen carrier and the wall of the specimen chamber then forming part of the magnetic circuit for the spot-forming magnetic field. In apparatus of this kind the specimen is arranged outside the mechanical boundary of the objective, so that comparatively large specimens can be examined and a high degree of mobility of the specimen can be achieved. The imaging in such apparatus is performed by the detection of electrons released from the specimen by the electrons in the primary beam. The detector for such released electrons is situated practically always at the top of the specimen, i.e. at the side where the primary beam is incident on the specimen. This means that in this manner only electrons released from the surface of the specimen are detected. In respect of their use these apparatus are considerably more flexible than said TEMs, constituting a significant advantage for the examination of large numbers of specimens, for example as necessary in the IC industry or more generally for the testing of materials. Apparatus of this kind, known as scanning electron microscopes (SEMs) generally may have a simpler construction; notably they are not arranged to operate with comparatively high acceleration voltages, their acceleration voltage being limited to the order of magnitude of 50 kV.
However, when SEMs are used it is often desirable to study regions which are situated deeper into the specimen. It does not suffice to detect electrons released from the surface of the specimen, but electrons have to be detected which originate from the volume of the specimen. This situation occurs also when information is to be extracted concerning individual layers in integrated circuits. To this end, the specimen is made to be so thin (for example, a thickness of between 60 nm and 400 nm), that electrons of an energy of the order of magnitude of some tens of keV can traverse the entire specimen. In that case the detector surface of the detector should be arranged underneath the specimen, so that electrons having the traversed the entire specimen can be detected.
In the apparatus which is known from the cited European patent application no. 0 348 992 the specimen to be examined is arranged outside the mechanical boundaries of the objective, so that the desired manipulability of the specimen is achieved. The detector surface is then situated underneath the specimen. This known apparatus is provided with a mask (referred to therein as xe2x80x9cinjection diaphragmxe2x80x9d and shown in FIG. 7) for transmitting or blocking electrons emanating from the specimen, which mask is arranged between the detector surface and the specimen. However, the known apparatus is arranged to accelerate the primary beam with a voltage of the order of magnitude of 75 kV or more, so that this cannot be called a SEM in the terms commonly used by those skilled in the art.
It is an object of the invention to provide a SEM of the kind set forth in the preamble of claim 1, in which notably specimens can be studied as they occur in the IC industry, it being possible to select a desired contrast mechanism upon imaging of the specimen and it being possible to detect crystal defects in crystalline specimens.
To achieve this, the apparatus according to the invention is characterized in that
the particle-optical apparatus is arranged to produce an acceleration voltage of no more than 50 kV,
that the mask is provided with at least one mask opening which is provided so as to be eccentrically situated in the mask,
that there are provided rotation means for rotating the mask during operation of the particle-optical apparatus.
The electrons transmitted by the specimen are scattered during transmission and hence leave the specimen at an angle other than zero relative to the direction of incidence. Because the mask is rotatable and provided with an eccentric opening, electrons from this dispersed beam can be selected from a given angular range. This angular range may concern an angle enclosed relative to the direction of incidence (by arranging the mask opening at a given distance from the optical axis), for an angle relative to a given direction in the specimen. Notably the latter possibility is important because it is thus possible to observe deviations which disturb the rotational symmetry of the specimen material. Rotation of the mask during the irradiation by means of the primary beam enables an angular distribution of the scattered electrons to be picked up.
The assembly formed by the specimen and the detector surface in an embodiment of the particle-optical apparatus according to the invention can be tilted through an angle relative to the optical axis, the detector surface being divided into at least two parts which can be electrically insulated from one another. These steps offer the additional advantage that the primary beam is not incident at right angles relative to the optical axis but at said angle. Because the detector surface is also tilted, it can be ensured that the non-deflected part of the beam is intercepted by one of the parts of the detector surface, and that the part of the beam which is deflected by the specimen is detected by another part of the detector surface. By subtracting the signals thus obtained from one another, information is acquired in conformity with the dark field observation method whereas information is acquired by way of the bright field observation method when the signals are added. Further angular selection, if any, can also be realized by means of the mask.
In another embodiment of the particle-optical apparatus according to the invention, the detector surface is displaceable in a direction parallel to the optical axis relative to the specimen location. By situating the detector surface nearer or further from the specimen, in combination with the selective effect of the mask a spatial selection of electrons transmitted by the specimen is also possible.
The mask opening in another embodiment yet of the particle-optical apparatus according to the invention has the appearance of at least one radial slit. Accurate angular selection (i.e. an angle relative to a given direction in the specimen) of the transmitted electrons can thus be performed. The accuracy of the selection can then be determined by the width of the slit.
The detector surface in another embodiment of the particle-optical apparatus according to the invention is subdivided into at least two mutually electrically insulatable parts, the mask being provided with at least two substantially diametrically situated mask openings. As a result of these steps, angular selection is possible in respect of scatter angle as well as in respect of an angle relative to a given direction in the specimen in combination, possibly also in combination with dark field or bright field observation.
In another embodiment yet of the particle-optical apparatus according to the invention the distance between the specimen and the detector surface is at the most 15 mm. This step is useful notably when use is made of an immersion lens because in that situation the specimen and the space therebelow are situated in the magnetic field of the lens. Consequently, the electrons transmitted by the specimen are subject to a given rotation which is dependent on the magnetic field strength and the distance traveled by the electrons underneath the specimen, which angular rotation obscures the information as regards the rotation of origin on the specimen. By keeping this distance small (while maintaining a given minimum distance in order to convert the scatter angle into a distance on the detector surface), the effect of such undesirable angular rotation is minimized.